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OCSP over DNS (ODIN)CableLabs858 Coal Creek CirLouisvilleCO80027USm.pala@cablelabs.comhttp://www.linkedin.com/in/mpalaSecurity
PKIRevocationOCSP
With the increase number of protocols and applications that rely on digital certificates
to authenticate either the communication channel (TLS) or the data itself (PKIX), the need
for providing an efficient revocation system is paramount. Although the Online
Certificate Status Protocol (OCSP) allows for efficient lookup of
the revocation status of a certificate, the distribution of this information via HTTP (or
very rarely) HTTPS is not particularly efficient for high volume websites without incurring
in high distribution costs (e.g., CDN).
In particular, this specification defines how to distribute OCSP responses over DNS
and how to define OCSP-over-DNS URLs in certificates.
The use of the DNS system to distribute such information is meant to lower the costs of
providing revocation services (by leveraging the distributed nature of DNS cache) and
increase the availability of revocation information (by providing an additional access
method for revocation information retrieval).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as
described in .
With the increasing number of highly available and highly utilized websites that
require secure communications to protect the flow of information from the server
to the client and the raising number of devices (IoT) that require strong authentication
capabilities, the need for a low-cost efficient approach to revocation information
availability is crucial.
The OCSP-over-DNS approach allows clients to determine the revocation status of digital
certificates by optimizing the delivery mechanism for revocation information
distribution to the client.
This transport protocol can be used in lieu of or in addition to other PKIX endorsed
transport mechanisms such as HTTP.
This specification addresses the problem of providing a highly-available distributed system
for OCSP responses .
This document defines the DNS records to be used for OCSP data publication and the
definition of additional URLs for the AuthorityInfoAccess (AIA) extension in certificates.
Currently there are three main options to retrieve the revocation information associated
with a digital certificates:
by retrieving the freshest CRLby querying an OCSP responder for a freshly computed responseby retrieving a pre-signed OCSP response from a web site
(typically a content distribution network or CDN)by verifying pre-computed OCSP responses embedded (stapled) during
the TLS negotiation (only in the TLS case, though)
All of these methods are based on the ability from the application to
extract URLs out of the CRL (CrlDistributionPoint) or of the OCSP
responder (AuthorityInfoAccess) from the certificate and query
(almost uniquely via HTTP/HTTPS, although supported protocols might
include LDAP and FTP) the corresponding server to retrieve the
required data.
This document focuses only on the definition of the required options
for providing OCSP responses over DNS as an alternative transport
protocol. The reliability and accessibility of DNS records (e.g.,
issues related to TCP vs. UDP DNS responses) are out of the scope of
this document.
In order to validate a certificate using OCSP-over-DNS, the client should
check the certificate for a DNS-based OCSP URI ("dns://") and then retrieve
the OCSP response from the DNS.
After this point, all procedures are to be performed according to the OCSP
protocol as defined in .
In particular, clients using OCSP-over-DNS, SHOULD:
Lookup the OCSP URI provided in the AIA of the certificate to be checked.
The format of the URI comprises the id-ad-ocsp identifier and a base URL
where the scheme (``dns://'') is used.
The format of the full URI is discussed in .
Retrieve the DNS record carrying the required OCSP response.
The OCSP DNS resource record (RR) is used to distribute a certificate's
revocation status to clients. The contents of the OCSP RR record are
described in .
The type value for the OCSP RR type is defined in .
The OCSP RR is class independent.
The OCSP RR Time to Live (TTL) should not exceed the validity period
of the OCSP response that is contained in the record.
The RDATA for an OCSP RR consists of a single field which carries the
DER encoded OCSP response for the identified certificate.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
+ OCSP Response Data /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The OCSP response should contain only one response that refers to the
certificate which contains that URL. Following this schema, the OCSP
DNS URIs within the AIA extension SHOULD be unique for each certificate
issued by a single CA.
This document uses a new DNS RR type, OCSP, whose value (TBD) was
allocated by IANA from the Resource Record (RR) TYPEs subregistry of
the Domain Name System (DNS) Parameters registry.
The time validity should reflect the frequency of updates in revocation
information (i.e., the TTL should not be set to expire after the OCSP
response expiration). In practice, as an operational matter, operators
SHOULD ensure that the records are published in a way that the TTL is
low enough that they expire from caches before the OCSP response
expiration.
The Authority Information Access extension, as defined in ,
provides information about the certificate in which the extension appears. In order
to specify the availability of OCSP responses over DNS, Certification Authorities
should use the OCSP accessMethod OID (id-ad-ocsp) and use "dns" as the transport.
Please note that, when using this accessMethod, the use of the dnsathority
in the specified URI is discouraged as this might reduce the benefits coming
from the caching infrastructure of DNS and, possibly, overload the referred
DNS server.
A DNS URL begins with the protocol prefix "dns"
and is defined by the following grammar, following the ABNF notation defined in
.
dnsurl = scheme COLON SLASH SLASH [target]
[QUESTION [ TYPE=rr_type ]
; target: is the dns entry for
; the lookup operation.
; rr_type: is the type of record
; to be retrieved. If not specified,
; the default type is OCSPRR
scheme = "dns"
SLASH = %x2F ; forward slash ("/")
COLON = %x3A ; colon (":")
QUESTION = %x3F ; question mark ("?")
TYPE = "type" ; the keyword ("type")
Although this specification does not mandate for any specific format for the
<target> component of the DNS URL, some examples are provided in
with the intent to illustrate, not define, the format.
In order to process the OCSP DNS URLs in a certificate, clients have to extract the <target>
and, if provided, the <type> of record from the URL. After that, client MUST query for the
specified record.
When the ``OCSPRR'' record type is used, the returned value MUST contain the DER encoded
OCSP response related to the certificate that the client is going to validate.
When using the issuing CA's DNS sub-domain in the DNS URL, the hex (or decimal) representation
of the certificate's serialNumber MAY be used as the hostname of the DNS URL.
When combined with the specific sub-domain of the issuing CA this provides a unique
entry that can be easily queried. For example, given that the sub-domain of the
issuing CA is "ca1.example.com", the resulting URL in the issued certificate can
be constructed as follows:
dns://04A3E45534A1B5.ca1.example.com?type=OCSPRR
Because the serialNumber of a certificate is guaranteed to be unique within a (single) CA,
different Certification Authorities MUST use different sub-domains when using this publication
algorithm to avoid collisions across different CAs.
However, in some environments, the serial number that will be used in the certificate to be
issued can not be pre-fetched and embedded in the AIA's DNS URL entry. In this case, the use of
a monotonically increasing or random integer number can be used instead.
In any case, it is important to notice that since the DNS entry is to be used "AS IS" by the
relying party that wants to fetch the OCSP response by using the DNS URL, other techniques (e.g.,
the use of prefixes for different issuing CAs combined with high-resolution clock entries
and small random or monotonic integer suffixes) can be implemented independently by different
Certificate Service Providers.
This document uses a new DNS RR type, OCSPRR, whose value (TBD) MUST be allocated by IANA from
the Resource Record (RR) TYPEs subregistry of the Domain Name System (DNS) Parameters registry.
Several security considerations need to be explicitly considered for the system administrators
and application developers to understand the weaknesses of the overall architecture.
It is important to highlight, however, that the following considerations are inherently derived
from the nature of the DNS infrastructure and that deployment of the DNSSEC protocol might
provide an efficient protection against them.
By lacking the ability to authenticate the originating server directly, the DNS (not DNSSEC)
protocol (both in TCP and UDP mode) is vulnerable to attacks where false responses are provided.
Although all the information stored in the OCSP RR is signed, the data returned to the client
could potentially be altered (e.g., by providing an empty or old response). This type of attack
can lead to the application's inability to retrieve the revocation information, thus this
approach is vulnerable to Denial of Service (DoS), Man-in-the-middle (MITM), and Reply Attacks.
As mentioned earlier, the deployment of DNSSEC can help in mitigating the described family of
attacks by providing a mean for the client (or its resolver) to verify signatures of the DNS
records themselves via the DNS keys.
This said, the use of DNS (instead of DNSSEC) is equivalent, from a security considerations
point of view, to today's deployment best practices for OCSP where pre-computed responses are
delivered by CDNs via HTTP (not HTTPS).
Therefore, the provisioning of OCSP responses via DNS does not lower or alter the security
considerations that apply to the use of OCSP.
Last but not least, because of the availability (in most cases) of independent DNS servers that
an application can query, the use of multiple requests to different DNS servers (for the same
DNS record) might be implemented as a mitigating measure in case an attack is suspected or
detected.
The authors would like to thank everybody who provided insightful comments and helped in the
definition of the deployment considerations. In particular, the authors would like to thank
Scott A. Rea for his support. We also would like to thank DigiCert and the initial discussion
and support for the initial idea. Last but not least, the authors would like to thank all
the people that expressed interest in implementing support for this proposal.
&rfc5234;
&rfc3986;
&rfc2119;
&rfc2560;
&rfc5019;
&rfc5280;
&rfc4501;